The overall aims of this proposal is to use in vitro and in vivo approaches to obtain basic, preclinical data regarding the effects of recombinant colony stimulating factors (CSFs) on hemopoietic cells and the hemopoietic microenvironment. In vivo studies have shown that CSFs stimulate myelopoiesis, but secondary effects on other lineages, particularly lymphopoiesis, have not been examined in detail. Studies using long-term bone marrow cultures (LTBMC) have demonstrated that CSFs inhibit lymphopoiesis in vitro, and the first aim will use LTBMC to investigate if this is due to direct effects on lymphoid cells or the lymphoid microenvironment. A fundamental question is whether the hemopoietic microenvironment can support steady state hemopoiesis when signals for one lineage are elevated, such as in CSF treatment. Accordingly, the second aim will examine if in vivo CSF administration depresses lymphoid development and that of other non-target lineages. If this does occur, bone marrow transplantation studies will determine if this is due to an inhibition of progenitor cell differentiation or to a depletion of progenitor cell populations by measuring the repopulating potential of marrow from CSF treated donors. If CSFs do inhibit lymphopoiesis, it may be of value to co-administer cytokines that stimulate that process. The third aim will test the in vivo potential of a novel factor, stromal cell derived lymphopoietic factor (SDLF-l), that induces pre-B cell production from precursors, in vivo. SDLF will be tested in both bone marrow transplantation and chemotherapy induced hypoplasia models. Regardless of their secondary effects, CSFs do stimulate myelopoiesis. The fourth aim, will examine if this is due to effects on the hemopoietic microenvironment in addition to direct actions on progenitors. For example, CSFs could stimulate stromal or accessory cells to produce additional cytokines, and this will be examined using stromal cell lines and LTBMC. Particular emphasis will be placed on whether the CSFs induce these cells to secrete mediators (IL-l, IL-4, IL-6) that could influence the outcome of cytokine therapy. This aim will also examine if drugs used to treat candidates for cytokine therapy further affect microenvironmental cell function. The final aim will determine if T cells have the potential to affect the pattern of hemopoiesis upon activation by major and/or minor histocompatibility differences. This is an issue of relevance and clinical marrow transplantation. T cells that are syngeneic, allogeneic or semiallogeneic at major/minor loci will be added to a new type of LTBMC in which myelopoiesis and lymphopoiesis proceed in parallel. If T cells do become activated and secrete CSFs or other mediators, their effects on both processes may be observed. Taken together, the data from these studies will provide additional information regarding CSF effects and their mechanism of action.